Memorandum from Raymond L. Murray to Dr. Clifford K. Beck
2 pp.
April 9, 1953

[page 1]


April 9, 1953

To: Dr. C. K. Beck
Subject: Antimony-Beryllium Neutron Sources
From: R. L. Murray
CC: Reactor Committee

At your request, I looked into the feasibility of using a Sb-Be neutron source in
the reactor. The purported advantage of such a unit is that it would be continually
activated by the thermal neutrons from the reactor by the reaction.
51Sb123 + on1 -> 52Sb124

The cross-section for neutron absorption in the Sb123 isotope, abundance 42.75%a
is 2.5 barns.b

The Sb124 isotope (60 day half-life) yields a variety of gamma rays of the order of
an Mev energy. These will cause photodisintegration of Beryllium to give neutrons.
These reactions are:
52sb124 -> 53Te124 + -1eo + [gamma]
4Be9 + [gamma] -> on1 + 4Be8

The Sb-Be sources supplied by Oak Ridgec of a 3/4" x 3/4" Sb cylinder,
surrounded by 11/64" of Be, and inserted in an Al can of 1/32" wall thickness. The
over-all diameter is 1.15 inches, the antimony weight is 35 grains. The activity
quoted for 120 day irradiation is 2.6 curies, in comparison with the saturation
activity of 3.5 curies. The number of neutrons per second of the freshly-prepared
source is given as 106/sec., which is marginal for reactor start up. Assuming, however,
that this number might be sufficient, the question to be answered is whether irradiation
in our flux would keep it at this strength. An estimate of this follows.

Let the saturation activity be identical to g, the generation rate in the ORNL reactor
flux [phi]. If the macroscopic absorption cross-section of Sb123 is [sigma], and the source
volume is V, then
g=[sigma] [phi] V

From the density of Sb (6.691 gm/cm³) the atomic weight (121.76 grams) and the
abundance, we find the number of Sb123 nuclei per cm³ to be 0.0142 x 10-4. Thus
[sigma] = 0.0355. Taking g = 3.5 curries (1.3 x 1011 d/sec), we find
[phi] = 6.9 x 1011 n/cm² sec.

A source irradiated at the center of the Raleigh Reactor at full power for an average
of 6 hours per day would experience an effective flux of about 1.25 x 1011 n/cm² sec.
(The peak central flux isd 5 x 1011).

[page 2]

To: Dr. C. K. Beck
Subject: Antimony-Beryllium Neutron Sources

A formula may readily be derived for the activity of a source of initial strength
Ao at any time t. It is
A = Aoe-[lambda]t + g1(1-e-[lambda]t)
where g1 is the generation rate in the reactor. Comparing RRR and ORNL flux,
g1 = 1.25x1011/6.9x1011 g; also by comparing saturation and delivered activities of the
source g = 3.5/2.6 Ao

Thus, g1 = 0.244 Ao and
A/Ao = e-[lambda]t (0.756) + 0.244

The time required for the activity to drop by a factor of two (an effective half life
found to be 94 days in comparison with the ordinary 60 day half life.

The following conclusions seem to be proper.


aNuclear Data, NBS 499 and supplements, Sept. 1950

bNeutron Cross-Sections, AECV 2040, May 15, 1952

cISOTOPES - Catalog and Price List, Oak Ridge National Laboratory

dNCSC 46 "Further Design Features of the Nuclear Reactor. . .", Jan. 1952